1. Field of the Invention
The present invention relates to a full duplex communication system, and more particularly, to an echo cancellation device in a full duplex communication system.
2. Description of the Prior Art
As technology advances, network applications have become more and more popular. Network bandwidth requirements are also increasing as the transmission speed of data transmission standards such as Ethernet have raised from 10/100 Mps to above 1 Gbps.
Each channel of a gigabit Ethernet system simultaneously performs transmitting and receiving operations. When the channel is transmitting, the signals received from the channel are affected by the transmission and this phenomenon is known as echo impairment. In order to reduce echo impairment in communication systems, an echo cancellation device is included within the transceiver. Please refer to FIG. 1 showing an equivalent circuit diagram of a line driver with a conventional echo cancellation device. The transmitter of the gigabit Ethernet system can be implemented to be a voltage mode transmitter which includes a line driver in the transmitter section 110. In FIG. 1, Rp is a matching resistor used for impedance matching, Zo is the channel's load, Vi is a transmit signal output from a transmitter section (line driver) 110, and Vo is an input signal of the twisted pair. The conventional echo cancellation device regards the channel's load Zo as a pure load resistor Re and the resistance of Rp is therefore equal to that of Re in order to match the impedance. From the circuit diagram shown in FIG. 1, the following formula can be obtained:
                    Vo        =                              Zo                          Zo              +              Rp                                ⁢          Vi                                    (        1        )            
The channel's load Zo is seen simply as the load resistor Re, thus Zo=Re. By substituting this relationship into formula (1), the following formula is obtained:Vo=½Vi  (2)
From formula (2), it can be seen that the influence at a receiver section 310 and caused by the transmit signal Vi output from the voltage-driving transmitter section 110 is ½ Vi. The conventional echo cancellation device is employed to generate an echo cancellation signal corresponding to the transmit signal. As such, the echo cancellation signal is ½ Vi and is followed by a subtractor to cancel the effect caused by the transmit signal in order to achieve echo cancellation.
However, a parasitic capacitance Ce effect is unavoidable in practical implementation. If the effective output impedance of the channel is seen purely as the load resistor Re, the echo cannot be effectively reduced to the lowest level. In other words, the conventional echo cancellation device 106 can only reduce DC component of the echo but not eliminate echo residue at high frequencies.